Control circuit for a laser diode

ABSTRACT

A circuit includes a laser diode and a switched-capacitance charge pump coupled to control the laser diode. The charge pump can include a capacitor and a switching circuit that is capable of triggering a charge and discharge of the capacitor. The switching circuit can include a switch and an inverting circuit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to French Patent Application No.1859035, filed on Sep. 28, 2018, which application is herebyincorporated herein by reference.

TECHNICAL FIELD

The present disclosure generally concerns electronic circuits and, inparticular, to a control circuit for a laser diode.

BACKGROUND

A laser diode is an optoelectronic component emitting monochromaticlight. Laser diodes are for example used to transport signals containinginformation over long distances.

To operate a laser diode, it is necessary to send regular current pulsesthereto. For this purpose, a laser diode generally comes along with acontrol circuit supplying such pulses.

It would be desirable to at least partly improve certain aspects ofknown laser diode control circuits.

SUMMARY

An embodiment provides a circuit for controlling a laser diodecomprising a switched-capacitance charge pump.

According to an embodiment, the charge pump is capable of emittingcurrent pulses at a frequency in the range from 10 to 800 MHz.

According to an embodiment, the charge pump comprises at least onecapacitor.

According to an embodiment, the at least one capacitor has a capacitancein the range from 10 to 500 pF.

According to an embodiment, the charge pump further comprises aswitching circuit capable of triggering the charge and the discharge ofthe capacitor.

According to an embodiment, the switching circuit comprises a firstswitch coupling a first electrode of the capacitor to a terminalreceiving a power supply potential.

According to an embodiment, the switching circuit further comprises aninverting circuit comprising an output terminal coupled to a secondelectrode of the capacitor.

According to an embodiment, the switching circuit is controlled by apulse train.

According to an embodiment, the pulses have a duty cycle in the rangefrom 5 to 40%.

According to an embodiment, the switching circuit further comprises asecond switch for balancing the charges of the laser diode.

According to an embodiment, the second switch has a first terminalcoupled to the cathode of the laser diode, and comprises a secondterminal receiving the power supply potential.

According to an embodiment, the second switch is an N-channel MOStransistor.

According to an embodiment, the second switch is controlled by the pulsetrain.

According to an embodiment, the second switch is a P channel MOStransistor.

According to an embodiment, the second switch is controlled by theinverse of the pulse train.

The foregoing and other features and advantages will be discussed indetail in the following non-limiting description of specific embodimentsin connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an embodiment of a laser diode control circuit;

FIG. 2 shows another embodiment of a laser diode control circuit; and

FIG. 3 shows an alternative embodiment of the circuit of FIG. 2.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The same elements have been designated with the same reference numeralsin the different drawings. In particular, the structural and/orfunctional elements common to the different embodiments may bedesignated with the same reference numerals and may have identicalstructural, dimensional, and material properties.

For clarity, only those steps and elements which are useful to theunderstanding of the described embodiments have been shown and aredetailed.

Throughout the present disclosure, the term “connected” is used todesignate a direct electrical connection between circuit elements withno intermediate elements other than conductors, whereas the term“coupled” is used to designate an electrical connection between circuitelements that may be direct, or may be via one or more intermediateelements.

The terms “about”, “substantially”, and “approximately” are used hereinto designate a tolerance of plus or minus 10%, preferably of plus orminus 5%, of the value in question.

FIG. 1 shows an embodiment of a circuit 10 for controlling a laser diodeD. Laser diode D comprises an anode A and a cathode K.

Control circuit 10 is a circuit of switched capacitance charge pumptype. Thus, circuit 10 comprises: a capacitor 12, a level shiftercircuit (LS) 13, an inverting circuit 14, a switch 16, and a switch 18.

Capacitor 12 comprises a first electrode coupled, for example,connected, to a node B, and a second electrode coupled, for example,connected, to a node C. Anode A of laser diode D is coupled, preferablyconnected, to node B. Capacitor 12 has a capacitance for example in therange from 10 to 800 pF, for example, in the order of a few hundreds ofpF. As a variation, capacitor 12 may be replaced with a plurality ofcapacitors in parallel.

Level shifter circuit 13 is a circuit outputting a signal Sdec shiftedwith respect to a signal S. Signal S is a signal carrying a pulse trainvarying between a reference potential GND, for example, the ground, anda positive power supply potential Vdd. The pulses have a duty cycle inthe range from 5 to 40% and have a frequency in the range from 10 to 800MHz. Signal Sdec varies between a low state equal to power supplypotential Vdd and a high state at least equal to twice 2*Vdd of powersupply potential Vdd.

Inverting circuit 14 comprises two switches 14A and 14B, for example,MOS transistors. Switch 14A is for example an N-type MOS transistor andswitch 14B is for example a P-type MOS transistor. Switches 14A and 14Bare coupled, for example, connected, in series. More specifically:

the source of switch 14A is coupled, preferably connected, to a terminalreceiving reference potential GND, preferably the ground;

the drain of switch 14A is coupled, preferably connected, to node C;

the source of switch 14B is coupled, preferably connected, to a terminalreceiving power supply potential Vdd; and

the drain of switch 14B is coupled, preferably connected, to node C.

Switches 14A and 14B are controlled by signal S.

Switch 16 is for example an N-channel MOS transistor. The source ofswitch 16 is coupled, for example, connected, to node B. The drain ofswitch 16 is coupled, for example, connected, to the terminal receivingpower supply potential Vdd. Switch 16 is controlled by signal Sdec.

Switch 18 is for example, an N-channel MOS transistor. The source oftransistor 18 is coupled, for example, connected, to a terminalreceiving reference potential GND. The drain of switch 18 is coupled,for example, connected, to cathode K of laser diode D. The gate ofswitch 18 is coupled, for example, connected, to node C. Node C beingcoupled to the output of inverter 14, switch 18 is controlled by theinverse of signal S.

Circuit 13, inverter 14, and switches 16 and 18 form a switching circuitof control circuit 10.

The operation of the control circuit comprises two phases. During afirst phase, capacitor 12 charges, and during a second phase, capacitor12 discharges into laser diode D. The two phases periodically alternateat the frequency of signals S and Sdec.

Capacitor 12 charges when signals S and Sdec are in a high state, thatis, when signal S is equal to power supply potential Vdd and when signalSdec is equal to twice power supply potential Vdd. The capacitance ofcapacitor 12 is adapted so that its charge time is shorter than theduration of a pulse of the pulse train of signals S and Sdec. In such aconfiguration:

switch 16 is on, and thus the potential at node B is equal to powersupply potential Vdd;

switch 14A is on and switch 14B is off. Thus, node C receives referencepotential GND; and

switch 18 receives, on its gate, reference potential GND and is thusoff.

Capacitor 12 discharges when signals S and Sdec are in a low state, thatis, when signal S is equal to reference potential GND and when signalSdec is equal to power supply potential Vdd. In such a configuration:

switch 16 is off;

switch 14A is off and switch 14B is on. Thus, node C receives powersupply potential Vdd; and

switch 18 is on.

The potential at anode A of laser diode D is then equal to twice (2*Vdd)power supply potential Vdd.

An advantage of this embodiment is that it enables to send currentpulses to anode A of laser diode D regularly, and more particularly atfrequencies in the range from 10 to 800 MHz.

FIG. 2 shows an embodiment of a circuit 20 for controlling laser diodeD. Control circuit 20 comprises the same elements as the control circuit10 described in relation with FIG. 1, which elements will not bedescribed again.

Control circuit 20 comprises, in addition to the components described inrelation with FIG. 1, a charge balancing switch 22.

Switch 22 is for example an N-type MOS transistor. The source of switch22 is coupled, preferably connected, to cathode K of laser diode D. Thedrain of switch 22 is coupled, preferably connected, to a terminalreceiving power supply potential Vdd. Switch 22 is controlled by signalSdec.

Control circuit 20 operates as follows.

During a charge phase of capacitor 12, switch 22 is on. Thus, anode Aand cathode K of laser diode D receive power supply potential Vdd.

The addition of switch 22 has no influence on the discharge phase ofcapacitor 12, since it is off in this case.

An advantage of this embodiment is that it enables to balance thecharges present in laser diode D during the charge of capacitor 12. Moreparticularly, this enables to fully discharge laser diode D. Indeed,without switch 22, in certain cases, laser diode D risks not fullydischarging. Fully discharging laser diode D further results in cuttingoff the optical emission of laser diode D more rapidly.

Another advantage of this embodiment is that the addition of anadditional switch may enable to attenuate spurious oscillations causedby the inductance of the conductors coupling capacitor 12 and laserdiode D. Indeed, switch 22 increases the resistivity of the controlcircuit, which attenuates spurious oscillations.

FIG. 3 shows an embodiment of a circuit 30 for controlling laser diodeD. Control circuit 30 is a variation of the control circuit 20 describedin relation with FIG. 2, where switch 22 is replaced with a P-type MOStransistor 32.

In this case, switch 32 is controlled by a signal inverse to signal S,for example, the gate of switch 32 is coupled, preferably connected, tonode C.

Various embodiments and variations have been described. It will beunderstood by those skilled in the art that certain features of thesevarious embodiments and variations may be combined, and other variationswill occur to those skilled in the art.

Finally, the practical implementation of the described embodiments andvariations is within the abilities of those skilled in the art based onthe functional indications given hereinabove.

Such alterations, modifications, and improvements are intended to bepart of this disclosure, and are intended to be within the spirit andthe scope of the present invention.

Accordingly, the foregoing description is by way of example only and isnot intended to be limiting. The present invention is limited only asdefined in the following claims and the equivalents thereto.

What is claimed is:
 1. A circuit comprising: a laser diode; and aswitched-capacitance charge pump coupled to control the laser diode, theswitched-capacitance charge pump comprising: a capacitor comprising afirst electrode coupled to the laser diode, a first switch coupledbetween the first electrode of the capacitor and a power supplyterminal, a diode switch coupled to the laser diode, the first electrodebeing coupled to the diode switch through the laser diode, and aninverting circuit comprising an input terminal coupled to a controlterminal of the first switch and an output terminal coupled to a secondelectrode of the capacitor and a control terminal of the diode switch.2. The circuit of claim 1, wherein the charge pump is capable ofemitting current pulses at a frequency in the range from 10 to 800 MHz.3. The circuit of claim 1, wherein the charge pump comprises a capacitorhaving a capacitance in the range from 10 to 800 pF.
 4. The circuit ofclaim 1, wherein the charge pump comprises a capacitor, the capacitorcomprising a discharge path through the laser diode and the diodeswitch.
 5. The circuit of claim 1, wherein the inverting circuit iscontrollable by a pulse train.
 6. The circuit of claim 5, wherein pulsesof the pulse train have a duty cycle in the range from 5 to 40%.
 7. Thecircuit of claim 1, wherein the charge pump further comprises a secondswitch for balancing the charges of the laser diode.
 8. The circuit ofclaim 7, wherein the second switch has a first terminal coupled to acathode of the laser diode and a second terminal coupled to a powersupply terminal.
 9. The circuit of claim 7, wherein the first switch andthe inverting circuit are controllable by a pulse train and wherein thesecond switch is an N-type MOS transistor controllable by the pulsetrain.
 10. The circuit of claim 7, wherein the first switch and theinverting circuit are controllable by a pulse train and wherein thesecond switch is a P-channel MOS transistor controllable by an inverseof the pulse train.
 11. A circuit for controlling a laser diode, thecircuit comprising: a capacitor with a first electrode configured to becoupled to the laser diode and a second electrode configured to becoupled to control a diode switch that is coupled between the laserdiode and a voltage node, the capacitor being configured to discharge bya flow of charge from the first electrode through the laser diode; aswitch coupled between the first electrode of the capacitor and a powersupply terminal; and an inverter comprising an output terminal coupledto the second electrode of the capacitor.
 12. The circuit of claim 11,wherein the capacitor has a capacitance in the range from 10 to 800 pF.13. The circuit of claim 11, wherein the switch and the inverter areconfigured to receive a pulse train.
 14. The circuit of claim 13,wherein pulses of the pulse train have a duty cycle in the range from 5to 40%.
 15. A circuit comprising: a laser diode having first and secondterminals; a first switch having a current path coupled between thesecond terminal of the laser diode and a reference voltage node; acapacitor with a first electrode coupled to the first terminal of thelaser diode and a second electrode coupled to a control terminal of thefirst switch, the capacitor having a discharge path through the laserdiode and the first switch; a second switch coupled between the firstelectrode of the capacitor and a power supply terminal; an invertercomprising an output terminal coupled to the second electrode of thecapacitor; and a level shifter having an input coupled to the input ofthe inverter and an output coupled to a control terminal of the secondswitch.
 16. The circuit of claim 15, further comprising a third switchhaving current path coupled between the power supply terminal and thesecond terminal of the laser diode.
 17. The circuit of claim 15, whereinthe first switch is configured to receive current pulses at a frequencyin the range from 10 to 800 MHz from the inverter.
 18. The circuit ofclaim 15, wherein the capacitor has a capacitance in the range from 10to 800 pF.